1. Field of the Invention
The present invention relates to an apparatus for playing back a rotary recording medium and, more particularly, to a tracking apparatus for the playback apparatus which reproduces video signals or other information signals out of a rotary magnetic recording medium such as a magnetic disk.
2. Description of the Prior Art
One of the major achievements in the imaging art is an electronic still camera system which uses an image pickup device, typically a solid-state imaging device or an imaging tube, and a recording apparatus of the type using a magnetic disk which is a relatively inexpensive and large-capacity recording medium. Specifically, the system is such that a desired image is purely electronically construed the form of electrical signals representative of a still picture to be recorded in a rotary magnetic disk, while the picture is reproduced by means of a television system, a printer, or the like.
A rotary magnetic recording medium applicable to such an electronic still camera system and others may comprise a disk which has a diameter as small as about 50 millimeters and allows fifty tracks to be recorded therein at pitches of about 100 microns, that is, with a track width of about 50 to 60 microns and a guard band width of about 50 to 40 microns. In a recording apparatus or a playback apparatus, the disk is rotated at a constant speed e.g., 3,600 revolutions per minute so that a video signal is recorded or reproduced on a field or a frame basis.
The problem with a recording medium adapted for such magnetic recording, particularly a magnetic disk, is that tracking errors are apt to occur due to compatibility with an apparatus, eccentricity, thermal expansion and others which are inherent in the recording medium. Tracking errors may cause a magnetic playback head to scan a track other than and adjacent to a desired one, resulting in crosstalk.
An approach heretofore proposed to solve the tracking error problem is a system in which, while a recording magnetic head records a tracking signal in a recording medium under application of a tracking servo, a playback head effects tracking servo utilizing the recorded tracking signal. It is impractical, however, to install a tracking servo mechanism which inherently requires accurate control in a camera or similar small-size and light-weight recording apparatus.
In light of the above, a guard band system or a frequency modulation (FM) azimuth system may be used for a recording scheme. The guard band or FM azimuth recording scheme is successful in preventing a playback head from scanning a track adjacent to a desired one or, if scanned, from picking up a signal prerecorded in the adjacent track, thereby compensating for some degrees of tracking error during reproduction.
Another known approach is a so-called envelope peak detecting autotracking control system. The principle of this system is such that during recording a record head is transported by a stepping motor at predetermined track pitches without effecting tracking servo and, during playback, a playback head detects an envelope of signals read out of each track so as to identify an optimum track based on the peak of the envelope, thereby effecting tracking servo.
In the envelope peak detecting autotracking system, a particular head position where the positive peak of envelope levels detected is determined by transporting a magnetic head a predetermined pitch to a first position to detect an envelope level there, then transporting it another predetermined pitch to a second position to detect an envelope level again, and then comparing the envelope level detected at the second position with that detected at the first position.
The comparison mentioned above may advantageously be implemented with a digital processing system, as has generally been done. For this end, a video signal read out of a track by a playback head is applied to an envelope detector and, then, envelope levels produced by the envelope detector are converted by an analog-to-digital converter to digital values to be applied to a digital processing system. Advantageously, only those envelope levels which are higher than a predetermined threshold should be compared so that detection errors due to noise and other disturbances within the system may be eliminated.
However, the envelope peak detecting autotracking control system has the drawback that since the head must be moved in opposite directions to track each track by detecting the peak of envelope levels, the response of the system is relatively slow.
Meanwhile, in a magnetic disk type rotary recording medium which is rotated at a predetermined constant speed, the linear velocity of a track relative to a head sequentially decreases from the radially outermost track toward the radially innermost track. Hence, the envelope levels of frequency signals which are sensed by the head and detected sequentially decrease from the radially outermost track toward the innermost one. Assuming that frequency signals prerecorded in the tracks of a recording medium lie in the same frequency band, the envelope level is generally dependent upon the condition of contact of the head with a track and upon the linear velocity of a track relative to the head and, therefore, the radius of a track, the latter being far more dominant than the former. Macroscopically, therefore, the envelope output level drops substantially in proportion to the radius of a track. In such a situation, there is an increasing demand for a tracking device which is capable of accurately and rapidly tracking tracks despite the dependence of the envelope level upon the head position.